The advantages of charging coking chambers with coal preheated to a temperature such that the coal is completely dry and below the temperature at which the coal is in a plastic state has long been recognized. Typically, the coal is preheated and pulverized, prior to being supplied to a pipeline that supplies the coking chambers, in a sealed assembly known as a disperser crusher. The coal is conveyed to the disperser crusher by hot gases and in the crusher is pulverized by a number of rotating hammers that are mounted on a common driven shaft that extends horizontally through a casing for the assembly. The disperser crusher is maintained at a pressure of approximately 2 psi above atmospheric and a temperature of about 500.degree. F. (approximately 575.degree. F. maximum) in response to the hot conveying gases and the sealed conditions of pipelines from the hot gas source to the coke oven.
To maintain the superatmospheric pressure within the disperser crusher, it is necessary for shaft seals to be provided in the casing wall for the drive shaft of the hammers. The shaft seal must be capable of withstanding the temperature on the shaft surface, which may rise to as much as 700.degree. F. because of the temperature in the disperser crusher and the frictional heating of the shaft. It is particularly important to keep coal particles that are flung in many different directions by hammers away from sealing members and surfaces to prevent wear. It is also important to prevent charring of the coal particles by air. If the coal particles become oxidized, they become charred. The accumulation of the oxidized particles, in particular, around the seal tends to destroy the seal integrity.
In the past, a seal for the shaft of a disperser crusher employed a single packing outside of the casing wall for the disperser crusher. The prior art packing included a minimum of three Teflon-asbestos packing rings, each having a radial thickness on the order of 5/8 inch, and longitudinally disposed along the shaft. The seal must be lubricated by grease, and to this end, the packing and lubricant must be located far enough from the exterior wall of the disperser crusher to prevent destruction of the lubricant by heat conducted through the shaft when the disperser crusher is operating at maximum temperature.
An annular chamber, provided between the seal and casing wall, is pressurized with air from an external source. The annular chamber is in fluid flow relation with the inside of the casing through a relatively close clearance hole around the shaft. The air flowing through the chamber ostensibly cleans the shaft to prevent migration of coal particles from the disperser to the packing. In actual practice, however, the prior art seal is not effective to maintain a pressurized condition within the disperser crusher for any prolonged time interval. None of the prior art seals has been able to maintain the desired pressure within the disperser crusher for more than four months, and some of the seals have been effective for only one week.
The ineffectiveness of the prior art seal can be attributed to many factors. In particular, the 5/8 inch external packing has been found to be excessively flexible and has caused the shaft to distort, resulting in a run out of from 0.015 to 0.20 inches. Charred coal from the disperser crusher migrates to and destroys the packing as well as a low coefficient of friction sleeve on the shaft, against which the packing abuts. The coal is charred because it is oxidized by air migrating through the clearance hole and because the internal pressure within the disperser crusher expels the charred coal particles from the disperser crusher axially of the shaft into contact with the coating and packing.
Another problem with the prior art seal is that shims are necessary to align the shaft. However, when the disperser crusher is shut down, for shaft alignment, the temperature of the shaft decreases, because of the lower temperatures within the disperser crusher. When the disperser crusher is stopped and the unit is cooling down the shaft sags and upon restarting, due to the sag in the shaft, misalignment between the shaft and packing occurs resulting in wearing of the packing.
A further problem with the prior art seal is that the packing requires lubrication by maintenance personnel. It has been found that the human factor involved in performing the lubrication frequently causes the packing to become dry, causing wear between the packing and shaft sleeve.
It is, accordingly, an object of the present invention to provide a new and improved shaft seal, particularly adapted to be utilized in connection with a disperser crusher that pulverizes hot coal particles.
Another object of the invention is to provide a new and improved seal for a shaft of a disperser crusher wherein the seal is not susceptible to destruction due to coal particles that are flung in many directions by the hammers of the disperser crusher.
Another object of the invention is to provide a new and improved seal for a shaft of a disperser crusher wherein the seal includes packings that are rigid enough to prevent possible distortion of the shaft carrying hammers.
A further object of the invention is to provide a new and improved seal for a shaft of a disperser crusher, which seal does not require lubrication.
A further object of the invention is to provide a new and improved seal for a shaft of a disperser crusher, which seal does not have an effect on the shaft alignment.